Theanine (L-glutamic acid-γ-ethylamide), which is known as an ingredient contributing to taste of green tea, has been elucidated to have various physiological actions such as a relaxing action, caffeine excitation suppressing action and hypotensive action. Thus, there is a great demand for theanine as a food additive. However, the content of theanine in green-tea leaves is extremely small. In the circumstances, a method for chemically synthesizing theanine has been desired.
Conventionally, in a known method for producing theanine, theanine is produced by reacting ethylamine with each of starting materials: N-trityl-L-glutamic acid-γ-benzylester obtained by protecting the γ-carboxyl group of L-glutamic acid by benzylation and the amino group thereof by tritylation (see Patent Document 1); N-substituted L-pyrolidone carboxylic acid (see Patent Document 2); a glutamic acid derivative obtained by protecting the α-amino group of glutamic acid by a Boc group and the carboxyl group thereof by an OtBu group (see Patent Document 3); and a glutamic acid anhydride obtained by protecting the α-amino group of the glutamic acid (see Patent Document 4); and thereafter, removing each protecting group.
However, the conventional method mentioned above has problems in that raw materials are expensive and product yields are low. It disadvantageously requires a number of steps and intricate operations associated with various chromatographic purification methods which lead to time-consuming work and high production costs. Due to these problems, it cannot be said that the conventional method is an industrially suitable production method. Furthermore, in consideration that the obtained theanine is used as a food additive, not only a method for producing highly purified theanine but also a method for producing theanine without using a harmful metal to human bodies has been desired.    [Patent Document 1] JP-A-5-70419    [Patent Document 2] JP-A-11-116542    [Patent Document 3] JP-A-2000-26383    [Patent Document 4] JP-A-2001-278848